Heat induced compounds in milk

Abstract:

Milk, preheated at 82°C for 30 minutes, was heated to 146°C
for four seconds (UHT-treated) and cooled to 5°C in a tubular heat
exchanger. Immediately after heat treatment, 20 gallons of heated
milk were vacuum distilled at 30°C in a semi-continuous, reduced
pressure glass apparatus. Twenty gallons of non-heated milk were
distilled in a similar manner to serve as a control. Continuous
liquid-liquid ethyl ether extractions were employed to recover the
compounds from the aqueous distillates.
Gas chromatography, mass spectrometry, infrared spectrophotometry
and odor confirmation were used to characterize the
compounds in the flavor concentrates. A technique for collecting
and transferring packed column gas chromatographic fractions to
capillary columns for mass spectral analysis was developed.
The following compounds were identified in UHT-treated milk (the underlined compounds appeared to result from the heat treatment):
the C₃, ₄, ₅, ₇, ₈, ₉, ₁₀, ₁₁, ₁₃ n-methyl ketones, the C₈, ₁₀, ₁₂
delta-lactones, acetaldehyde, hexanal, benzaldehyde, furfural,
phenylactaldehyde, vanillin, the C₆, ₈, ₁₀ n-alkanoic acids, ethanol,
oct-1-en-3-ol, n-heptanol, 2-butoxyethanol, diacetyl, maltol,
acetophenone, ethyl acetate, benzothiazole, toluene, naphthalene,
a dichlorobenzene, a trichlorobenzene, methyl iodide, benzonitrile
and chloroform.
The following compounds were identified in non-heated milk:
C₃, ₄, ₅, ₇, ₉ n-methyl ketones, C₁₀, ₁₂ delta-lactones, hexanal,
benzaldehyde, C₆, ₈, ₁₀ n-alkanoic acids, ethanol, diacetyl,
ethyl acetate, methyl palmitate, diethyl phthalate, a dichlorobenzene,
a trichlorobenze and methyl iodide.
The concentration of diacetyl in UHT-treated and non-heated
milk was determined by a modified gas entrainment, on-column
trapping GLC technique. The amount of diacetyl in non-heated milk
was 3 ppb while the amount in the UHT-treated was 38 ppb. The
diacetyl concentration of UHT-treated milk decreased approximately
40% over 16 days storage at 4°C. The average flavor threshold for
diacetyl in milk was found to be 12 ppb. It seems therefore that the
UHT-treatment increased the diacetyl concentration from a subthreshold
level to above the average flavor threshold. It is suggested
that diacetyl contributes to the "rich", "heated" note in the flavor of
heated milk.